Modern wind turbines are used to produce electricity. They are often very large structures with blades of up to and in excess of 60 meters and made from fibre-reinforced polymer structures, such as shell elements. These wind turbines are provided with control devices which may prevent an overloading of the wind turbine and the blades at wind gusts and high wind speeds. Such control devices can also be used to slowing the rotor down and bringing it to a complete halt, if the wind speed becomes too high. In addition to these devices the turbine may comprise a braking system arranged in communication with the main shaft of the wind turbine.
The control devices may be formed of pitch-controlled blades mounted such on the hub that they are able to turn about their longitudinal axis. The blades may thus be continuously adjusted to provide the lift rendering the desired power. In so-called stall-controlled wind turbines the blades are fixedly mounted on the hub and thus unable to turn about their longitudinal axis. In this case, the stall properties of the blades are used to reduce the aerodynamic lift and thus the power output.
The lengths of wind turbine blades have increased over the years and may now as previously mentioned exceed 60 meters. The increase in length also leads to increased mechanical loads from strong winds and from fluctuations in the wind. The loads are primarily caused by changes in the local inflow or turbulence. This in turn causes pressure changes over the surface of the wind turbine blade, which finally changes the loads on the blade. Typically, the loads are measured by use of strain gauges, which are mounted on the blade or imbedded in the shell structure of such a blade. Such strain gauges may for instance be resistive or in form of optical fibres, e.g. provided with Bragg gratings. However, once the effect on the load is detected, it is already too late to fully compensate for the load changes. To do so, information on the changes in the inflow or turbulence is needed beforehand, i.e. before these inflow changes impact the wind turbine blade. This may for instance be obtained by arranging pitot tubes at the leading edge of the blade in order to probe the wind velocity. However, such pitot tubes influence the flow characteristics of the blade, and furthermore pitot tubes may act as a lightning receptor, thus attracting lightning strikes potentially damaging the wind turbine blade. Light Detection And Ranging (LIDAR) systems may be used for non-invasive probing of wind velocities upwind of the wind turbine and have been proposed used in connection with compensating for yaw errors or keeping the rotational speed of the rotor substantially constant by pitching the individual wind turbine blades. The LIDAR system is typically proposed to be mounted on top of the nacelle of the wind turbine and probes wind speeds in a probing region located hundreds of meters in front of the wind turbine.
U.S. Pat. No. 6,320,272 describes a wind turbine provided with a LIDAR system on top of the nacelle. The LIDAR system is utilised for anticipating the wind speed upwind of the wind turbine and pitching the blade in order to obtain a substantially constant rotational speed of the rotor.
US2006140764 discloses a LIDAR system mounted in the hub of a wind turbine. The LIDAR has a viewing direction, which is inclined to the rotational axis so that the rotation of the hub ensures a scanning in front of the rotor.
US 2007075546 discloses a wind turbine provided with a LIDAR system for measuring wind speeds in front of a portion of a wind turbine blade. The LIDAR is mounted in the hub or at a base of the tower.
However, the wind is non-uniform over the length of a wind turbine blade due to turbulence, tower shadow, wind shear, yaw errors, wake effects and the like. This non-uniformity causes varying forces along the blades, which is turn cause fatigue loads and extreme loads on the wind turbine. These phenomena become even more pronounced as the wind turbine blades become longer and longer. To compensate for such fluctuations it is not sufficient to obtain a single measurement hundreds of meters in front of the rotor.
WO2007045940 discloses a wind turbine blade having a variable aerodynamic profile. The document further mentions that a laser anemometer may be used to measure the wind speed in front of the blade and that an anemometer may be arranged near the tip of the blade. However, the document does not provide any details on how such an anemometer should be mounted to the blade and where exactly the anemometer should probe the wind speed.
WO2004075681 discloses a method of controlling aerodynamic load of a wind turbine based on a local blade flow measurement. The document mentions that a laser Doppler anemometer may be utilised to measure the instant angle of attack or the wind velocity. However, the document does not provide any details on where and how to arrange the anemometer.